The invention relates to a method for testing the leak-tightness of a fuel supply system of a motor vehicle, including a fuel tank, in the interior space of which there is provided a volume-changing element, the so-called compensation volume of which is normally connected, in particular with the interposition of an accumulator unit for gaseous fuel constituents, to the surroundings. The interior space of the fuel tank, which can be filled with fuel for a consumer, is connectable to the surroundings by way of a valve unit which is normally open during the filling of the fuel tank and in the event of an exceedance of a positive-pressure threshold value in the region of up to 100 mbar and in the event of an undershooting of a negative-pressure threshold value. The “positive pressure” and “negative pressure” relate to the difference between the pressure in the tank interior space and the ambient pressure. The valve unit is otherwise closed. A fuel supply system of said type is described in the international patent application with the file reference PCT/EP2015/065891.
The legal requirements placed on the prevention of emissions from motor vehicle tank systems have over time become extremely high. Under virtually all conceivable circumstances, taking into consideration different country-specific regulations (e.g., the different nations of the world issue different regulations), practically no gaseous fuel constituents (e.g., these are normally hydrocarbons) should pass out of the interior space of the tank into the surroundings. As is known, particularly large quantities of such gaseous fuel constituents are encountered during the filling of the fuel tank with fresh fuel, but also as a result of a temperature increase in the case of the motor vehicle being at a standstill for a relatively long period of time. In conjunction with the latter, a person skilled in the art is familiar with the expression “diurnal losses”, that is to say the losses of gaseous fuel constituents, or the quantity of gaseous hydrocarbon emissions, that must be discharged from the tank interior space owing to temperature fluctuations (for example resulting from the change between day and night) when the motor vehicle is at a standstill for a relatively long period of time in order to prevent the generation of inadmissibly high positive pressure in the tank interior space. It is however also necessary to discharge hydrocarbon emissions that form in the tank owing to temperature changes during the operation of the motor vehicle. In each case, the adherence to vapor pressure equilibrium in the tank leads to such emissions. As is known, at least these emissions that do not occur during the filling of the fuel tank are temporarily stored in an activated carbon filter or the like, which in the present application is generally referred to as accumulator unit for gaseous fuel constituents, until an internal combustion engine which is supplied with fuel from the fuel tank (and which is normally provided as a vehicle drive unit) is suitably set in operation such that said accumulator unit can then be purged. The fuel constituents temporarily stored in the activated carbon filter are supplied to the internal combustion engine for combustion. For the sake of completeness, it is also pointed out that, in specific countries, it is conventional for hydrocarbon emissions that occur during the filling of the tank either to be suctioned off during the filling process at the refueling station itself (for example, “ECE system” in the European Union) or to likewise be stored in a then adequately large-dimensioned activated carbon filter or in a corresponding storage unit (for example, ORVR (Onboard Refueling Vapor Recovery) in the USA).
With regard to an avoidance of the above-mentioned “diurnal losses” or other operational hydrocarbon emissions, pressure tanks have already been proposed, or are already in series production, in which a correspondingly high positive pressure (and a low negative pressure) can prevail such that no discharge of fuel vapors from the tank need occur other than in extreme exceptional circumstances, but such pressure tanks are highly cumbersome. Also known are measures for reducing the formation of gaseous fuel constituents in the tank interior space, in particular in conjunction with a filling process of the fuel tank, as in, for example, U.S. Pat. No. 5,460,135. Specifically, for the USA, it is the case that all gaseous hydrocarbon emissions that occur during the filling of a vehicle fuel tank must be temporarily stored in a storage unit for gaseous fuel constituents situated on board the vehicle. To reduce the quantity of such gaseous hydrocarbon emissions, said cited document provides, in the fuel tank, a flexible air bladder which exhibits its minimum volume when the tank is completely filled with fuel and which is continuously filled with ambient air during the extraction of liquid fuel from the tank, whereas an evacuation of said air bladder is prevented. Therefore, it is self-evidently possible for only a smaller quantity of fuel vapors to form above the liquid level in the tank than in an otherwise identical tank without an air bladder of said type. In conjunction with refilling of the tank, said air bladder is then evacuated into the surroundings through the storage tank or activated carbon filter.
The international patent application with the file reference PCT/EP2015/065891 as cited above has described a fuel supply system, the functional principle of which is based on the recognition that, owing to the vapor pressure equilibrium in a fuel tank which is almost completely filled with liquid fuel, fewer gaseous fuel constituents form above the liquid level than in the same fuel tank when the latter is only, for example, half full or less. Consequently, with a flexible air bladder or the like which is provided above the liquid level formed by liquid fuel in the tank interior space, the quantity of fuel constituents that transition into the gaseous state in the tank interior space can be reduced. In said patent application, and also in the present case, reference will be made below not to an “air bladder,” but more generally to a volume-changing element which, arranged in the interior space of the tank, has or encloses a variable compensation volume which is or can be connected to the surroundings. Accordingly, the volume-changing element can breathe, as it were, that is to say can become larger or smaller in accordance with demand and thus react to changed boundary conditions.
A (further) legal licensing requirement for motor vehicles for example in the United States of America is that the entire region of the fuel supply system in which gaseous fuel constituents can be present is automatically checked for leak-tightness at regular intervals. Normally, such leak-tightness tests are performed by way of an electronic control unit during every driving cycle or every second driving cycle of the motor vehicle. Here, a leak, for example in the form of a hole with a diameter of only 0.5 mm, must be immediately detected, and the existence of such a leak must be indicated to the user of the motor vehicle. For this purpose, in the customary prior art, in that region of the fuel supply system in which gaseous fuel constituents can be present, the pressure and the temperature (of the gases situated in said region or, in the case of sensors being arranged in an upper region of the fuel tank, if the latter is completely filled, also of the liquid fuel situated there) are detected by way of suitable sensors over a certain time period. Said region also includes the fuel vapor intermediate store.
It is an object of the present invention to specify, for a fuel supply system, e.g., according to the above-cited international patent application with the file reference PCT/EP2015/065891, a suitable method for testing the leak-tightness thereof.
This and other objects are achieved by a method for testing a leak-tightness of a fuel supply system of a motor vehicle, where, by way of a gas-conveying device, a differential pressure in relation to the surroundings is generated in the tank interior space with the volume-changing element connected to the surroundings, and, through switching of a suitable shut-off valve, is held and monitored over a certain (or predetermined) period of time or inspected after a certain (or predetermined) period of time. If the differential pressure still exceeds a certain (or predetermined) threshold value after this time period, adequate leak-tightness of the fuel supply system is inferred. Conversely, if the differential pressure does not exceed the threshold value a leak can be directly inferred, though there is also the risk of incorrect measurements or other disruptive influences. For such a reason, in an advantageous refinement of the invention, it is contemplated that, if the differential pressure lies below the stated threshold value after said time period, the above-described testing routine is performed again, and then inadequate leak-tightness of the fuel supply system is inferred only if a particular number of such testing routines which show inadequate leak-tightness of the fuel supply system have been performed in succession. Said particular number preferably lies in the range of, e.g., 2 to 4.
For the further explanation, it is firstly pointed out that, with regard to the features of a fuel supply system relevant for the present invention, reference is made to the full content of the cited international patent application with the file reference PCT/EP2015/065891, which does not constitute a prior publication, that is to say it is the intention for the entire content of said document to be incorporated into the content of the present patent application. Therefore, below, an exemplary embodiment of the present invention will be described on the basis of a fuel supply system which is illustrated in FIG. 1 of, and described in detail in, the above-cited international patent application.
According to an embodiment of the invention, a leak-tightness test of a fuel supply system of said type or of a fuel supply system similar thereto is performed in that, by way of a gas-conveying device (in particular in the form of an air pump driven by electric motor), a differential pressure in relation to ambient pressure is generated only in that part of the test-subject region of the fuel supply system which also has to be tested for leak-tightness in a conventional fuel supply system that has no volume-changing element in the tank. This in particular also includes the fuel tank itself, such that a differential pressure in relation to ambient pressure is generated in the fuel-fillable volume of said fuel tank. The gas-conveying device is thereupon deactivated and, as a result of suitable valves being closed, said generated differential pressure (for example negative pressure) is, in effect, confined. If said negatively pressurized part of the fuel supply system is leak-tight, then said differential pressure should exist for a certain (predetermined) period of time, for which reason the value of said differential pressure (for example negative pressure) is queried or checked again at least after the expiry of a certain (predetermined) time period.
Said part of the fuel supply system which is charged with a differential pressure (in relation to ambient pressure) for the leak-tightness test may now have a leak either in a line section or at the fuel vapor buffer accumulator or at some other location and be connected to the surroundings there, whereby said differential pressure would be depleted. Said part of the fuel supply system which is charged with a differential pressure can however also be connected to the surroundings via the volume-changing element, which according to an embodiment of the invention itself is connected to the surroundings, if a wall or casing wall (or the like) of the volume-changing element, which delimits the compensation volume of said volume-changing element and separates said compensation volume from the fuel-fillable remaining space of the tank, exhibits a leak. It would then specifically be the case that pressure equalization would occur via or through said volume-changing element. At the same time, and without additional effort, it is thus possible for the leak-tightness of the volume-changing element itself to be checked in a particularly simple manner.
If a change in the differential pressure value within said certain time period then lies below a particular suitably predefined pressure magnitude, the test-subject region of the fuel supply system can be regarded as exhibiting adequate leak-tightness; otherwise, there is a substantiated suspicion of a leak. That region of the fuel supply system for testing or tested by way of said method in this case encompasses the tank including the volume-changing element and a basically conventional accumulator unit for fuel vapors and the purge line thereof.
As has already been mentioned, the above-mentioned substantiated suspicion of a leak can be further confirmed if the testing discussed thus far, specifically the generation and monitoring of the maintenance of differential pressure, which may furthermore be a positive pressure or negative pressure in relation to ambient pressure, is performed several times in direct succession, resulting in a suspicion of a leak every time. For example, after three successive testing routines according to the invention which do not show adequate leak-tightness, it is possible to assume that a leak is present despite the possibility of incorrect measurements or adverse boundary conditions. By contrast, if adequate leak-tightness has been detected only once, adequate leak-tightness can be inferred with certainty, because, put simply, it is much more complicated to realize leak-tightness than a leak. Furthermore, the method according to the invention may be performed whenever suitable or (for example legally) required.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.